1. Field of the Invention
This invention relates to a method of and apparatus for severing a refractory material, e.g. a glass sheet along a score.
2. Discussion of the Art
There are taught in the art various techniques for severing glass sheets, e.g. in U.S. Pat. Nos. 4,088,255 and 4,136,807 members are urged against a glass sheet to apply bending moment forces about a score to sever the sheet and in U.S. Pat. Nos. 3,695,497 and 3,730,408 a heat source, e.g. a delineated infrared source and hot air heaters, respectively, are used to create a thermal score or heat path in a glass sheet afterwhich bending moment forces are applied about the heat path to sever the sheet. Although each of the techniques taught in the above-mentioned U.S. patents are acceptable there are limitations. For example a limitation of the techniques taught in U.S. Pat. Nos. 3,695,497 and 3,730,408 is the time required to concentrate the heat in the sheet to set up stress fields to create the heat path. Further, a limitation common to each of the above-mentioned techniques is that the glass sheet is contacted to apply the severing bending moment forces and this contact can mar the glass surface.
The limitations of the above techniques are not present in the technique of U.S. Pat. No. 3,695,498. In general, a delineated infrared source is positioned at one focus of an elliptical reflector to cause the energy to be concentrated in a glass piece along an intended line of fracture. The heat generates a stress field in the glass sheet sufficient to sever the sheet. The limitation of this technique is the energy required to set up the stress field and the control of the stress field to provide a fracture along a predetermined path of fracture. U.S. Pat. No. 3,754,884 teaches the severing of a glass sheet from a ribbon heated to a temperature corresponding to the strain point of the glass. Although this technique is acceptable for practice on a glass ribbon during its manufacture it is not acceptable for severing glass sheets which must be heated in order to practice the severing technique. U.S. Pat. No. 3,520,455 teaches the severing of ceramics by applying a boron nitride coating along an intended path of cut and heating the boron nitride to sever the ceramic piece. The limitation with this technique is the need of the boron nitride coating and a uniform coating thereof to provide uniform thermal stresses in the ceramic piece.
U.S. Pat. Nos. 3,756,482 and 3,795,502 each teach in one form or another, the severing of a glass sheet by imposing a score in the sheet followed by heating the score to propogate the score through the sheet to sever the sheet. The limitation of this technique is that the score is propogated through the glass by the use of heat which sets up stress fields and may result in cut edge defects. Further, if the glass sheet has scores transverse to those being propogated by the heat, the application of heat may affect the cut edge of the opened transverse scores.
As can now be appreciated, it would be advantageous to provide techniques for severing a glass sheet along a score using noncontacting bending moment forces that do not have the limitations of the above-mentioned art.